The present invention relates to a new inhibitor composition useful for preventing corrosion by solvents used in treating sour gas streams and to the inhibited solvent.
The conditioning of naturally occurring and synthetic gases by absorbing acidic gases such as CO.sub.2, H.sub.2 S, COS, and HCN in an absorbent solution has been practiced commercially for many years. Gases such as feed gas for an ammonia plant, natural gas, and flue gas are examples. Aqueous solutions of various compounds such as alkanolamines, sulfolane (tetrahydrothiophene-1,1-dioxide), potassum carbonate, and mixtures of two or more of these have been used for the purpose. The water may be replaced in part or in whole by a glycol. All of these systems are plagued by corrosion of metal equipment which can be caused by products of degradation of the absorbent, by acidic components, or by products of reaction of these acidic components with the absorbent. For example, although aqueous alkanolamine itself is not particularly corrosive toward iron and steel equipment, it becomes highly corrosive when there are dissolved sour gases present, particularly when it is hot. To combat this problem, various metal compounds have been used alone or in combination with other compounds as corrosion inhibitors, for example, compounds of arsenic, antimony, and vanadium. While such metal compounds are effective inhibitors of CO.sub.2 -promoted corrosion, they are ineffective when H.sub.2 S is present. These metal compound inhibitors also have the disadvantages of low solubility in most gas conditioning solutions and of relatively high toxicity. The latter property is particularly undesirable since it affects both the handling of the solvent and the disposal of waste material.
An organic inhibitor system for inhibiting corrosion of ferrous metals by sour gas conditioning solutions used primarily to remove CO.sub.2 which comprises the combination of a quaternary pyridinium salt and a thio compound which is a water-soluble sulfide, thiocyanate, or an organic thioamide in a weight proportion of one part of pyridinium salt to about 0.001-10 parts of thio compound is described in our concurrently filed application S. N. 782,156 entitled Inhibitor for Gas Conditioning Solution. However, this organic inhibitor combination is less effective when the sour gas contains substantial amounts of H.sub.2 S as well as CO.sub.2.